The invention relates to a circuit arrangement for preventing unauthorized access to a communication system which is protected by a subscriber-specific password. Such passwords are assigned as proof of identity of a person authorized to access a communication system which is protected from unauthorized access to the authorized person in addition to a general indentification. These passwords must only be known to the authorized person and to the place where the decision about the access is taken. Before access is given it is checked whether there is indeed agreement between the assigned identification and the password.
As a rule all the assigned secret passwords of all the authorized persons are stored in a data bank of the place(s) where the decision about the access is taken. After an identification has been received, the deciding place waits for a password which agrees with the assigned password checked in its data bank. When there is agreement between the passwords the person requesting access is indeed authorized and indentified as such.
Whether such a system is protected from unauthorized use depends fundamentally on the extent to which the content of the password memory can be protected from unauthorized reading or changes. The risk that a data bank is read by unauthorized persons has significantly increased because of the enormously increased use of home computers and a corresponding wider knowledge in a vast number of subscribers. Successful efforts by computer-aided systematic or random trials to find a secret password are ever increasing.
In this situation the measure according to the invention becomes operative. The invention has for its object to counteract by appropriate circuit measures a potential manupulator who tries to obtain in a fraudulent way access to protected information, for example in a data bank, a mobile radio network, a converter network, using a series of guessed passwords.
This object is accomplished by the circuit arrangement described herein.
Because of the progress in modern semiconductor storage techniques, the overall circuit arrangement requires only very little space. The circuit arrangement is frequently provided in a further embodiment of the invention before each selector stage of a switching system or before each accessible channel of a mobile radio system. Consequently, the intended blocking of a system is within narrow limits.
To increase the protection, all the elements of the circuit arrangement are provided unaccessibly on a support and surrounded by an unaccessable envelope. Direct material access to the store results in the destruction of the storage arrangement. Electrical access extends, when there is no agreement between the passwords applied to the arrangement, the access time for the next scanning operation of the stored data by a factor of 1.times.10.sup.8. Theoretically, a 16-bit password may require up to 2.sup.16, i.e. 65.536 access trials with extended access time, to obtain one single access.
In the above example the blocking period after non-agreement would amount to 1.times.10.sup.8 multiplied by a 500 ns storage (control) access times i.e. 50 s. For 65.536 possible trials this would mean a time equal to 65.536 times 50 s, i.e. 910 hours or 38 days for obtaining one single successful access. An average period of 10 to 14 days may be assumed to be a realistic time required for getting successful access once.